This function draws a polar diagram of angle map such as aspect or flow directions. You can access it by typing d.polar on the command line in GRASS. Just fill in the required information (the input map) and name of the EPS output file (optional, you can also choose to have it displayed on screen).
Alternatively, fill in all on the command line, like in the example below. In the example, I first plot the distribution of the slope aspect in an area around Mount Kilimanjaro in Tanzania (Figure 1).
d.polar map=aspect@PERMANENT eps=aspect_1.eps
This gives the polar diagram in Figure 2. As you can see, there is not a clear prevailing direction of the slope aspect. The strong spikes in NW, SW, SE and NE directions indicate an over representation of those angles, which might be because the DEM used to calculate the slope aspect map is an integer map.
Next, I plot the slope aspect distribution of the Afromontane dry transitional forest in the area (the dark green area in Figure 1). To do this, I mask out all other areas and run d.polar again.
r.mask input=pnv_vecea2_agg@ConsStat maskcats=10 d.polar map=aspect@PERMANENT eps=aspect_2.eps
As you can see in Figure 3, this simple exploratory analysis shows that this type of forest occurs more often on the southern slopes (I know, I could have told as much directly from figure 1).
This becomes even clearer when combining the two figures in Figure 4. I did not find an option to create this combined polar diagram directly, so I used GIMP to combine the two diagrams above.